1. Field of the Invention
The present invention relates to video tape recorders (VTR), and more particularly to the so-called time lapse VTR that allows long-time recording by carrying out field extraction recording of a video signal at a predetermined period.
2. Description of the Background Art
The so-called time lapse VTR is developed wherein a video signal is sequentially recorded on a tape at a ratio of one field to every N fields (field extraction recording) while running the tape at a speed of 1/N (N is an integer) the standard tape running speed at the time of recording, and wherein a video signal on a tape is reproduced while running the tape at the aforementioned standard tape running speed to obtain a high speed reproduced picture at the time of reproduction. Such a time lapse VTR allows long-time recording by the above-described field extraction recording in comparison to the case where a standard video signal recording is carrying out at a standard tape running speed. It is used in various fields requiring long-time recording such as for a security monitor camera.
An example of such a time lapse VTR is disclosed in Japanese Patent Laying-Open No. 62-166671. In this conventional example, a video signal is extracted at a ratio of one field to every three fields (a three-field period) such as the first field, the fourth field, the seventh field, . . . , to be recorded on a magnetic tape. Therefore, a recording pattern is formed of an alternate arrangement of odd number fields and even number fields on a magnetic tape. That is to say, adjacent tracks on a magnetic tape can be formed using separate heads having azimuth angles differing from each other.
Although such a conventional time lapse VTR allows recording of a long term (3 times) by a field extraction recording of a 3-field period, there is no particular definition of the tape running speed with respect to the standards of a VHS system, and is defined as only to be constant. Therefore, direct compatibility was absent, and had a problem that a video signal recorded on a magnetic tape with the above-described time lapse VTR could not be reproduced with a conventional VTR of the VHS system.
A conventional time lapse VTR is generally provided with a 4-head cylinder of double-azimuth mainly for reproduction. A 4-head cylinder of double-azimuth includes a pair of heads A+ and A- having azimuth angles differing from each other and arranged opposed to each other by 180.degree. along the circumference of the cylinder, and another pair of heads B+ and B- having azimuth angles differing from each other (head B+ having an azimuth angle identical to that of head A+, and head B- having an azimuth angle identical to that of head A-) and arranged opposed to each other by 180.degree. along the circumference of the cylinder. Head B- is arranged in the proximity of head A+, and head B+ is arranged in the proximity of head A-.
In the above-described time lapse VTR, still-reproduction or slow-reproduction may be carried out by an intermittent reproduction in which the same track is traced for a plurality of times at the time of reproduction. In this case, the same track, i.e. one track formed by a recording head of one azimuth angle, must be traced by a head having the same azimuth angle continuously for a plurality of times. Therefore, heads of the same azimuth angle must be provided at positions opposing to each other by 180.degree. along the circumference of the cylinder (in the above-described embodiment, the pair of heads A+ and B+, and the pair of heads A- and B-). Therefore, a 4-head cylinder of double-azimuth is provided at least for reproduction.
At the time of reproduction, the envelopes of reproduced outputs from respective heads are compared, and a head from which a greater reproduction output is obtained is selected as a reproduction head. More specifically, the outputs of a pair of heads arranged adjacent to each other with different azimuth angles and tracing a track (the pair of heads A+ and B-, or the pair of heads A- and B+) are constantly compared. When the relation in magnitudes of the outputs is reversed, the reproduction head is switched at any time, even during a tracing of a track of one field.
This results in a reproduced video signal having a discrete step in level at the time of switching the reproduction head, leading to a problem that noise is generated on the reproduction screen.